The present invention concerns a method for producing construction panels consisting of reinforcing organic fibers, preferably cellulose fibers, and of at least one organic binder such as cement and possibly of fillers, wherein the crude panels are subjected to air hardening by being stored in stacking structures.
Of all the materials which are known and available for roofing and cladding, asbestos reinforced corrugated cement sheets and boards are among the best, since they possess good strength and their weathering characteristics give them a long life. A basic disadvantage of asbestos-cement sheets is the fact that they contain asbestos, a fibrous material the dust of which is thought to constitute a health hazard, and the industry is therefore constantly seeking alternatives to this materials.
No single material has been found with characteristics which satisfactorily match all those of asbestos, and it has thus been proposed to use mixtures of materials, e.g. mixtures of fibrous materials such as cellulose and glass fibers.
The state of the art of Asbestos-Cement Board and the chemistry of the portland cement bonding thereof may be ascertained by reference to the Kirk-Othmer ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, 2nd Edition (1970), Volume 21, pp 607 and 608.
The state of the art of non-asbestos cement panels may be ascertained by reference to U.S. Pat. Nos. 3,969,567; 4,101,335 and 4,428,275, and European Patent Specification 68,741 published Mar. 19, 1986, the disclosures of U.S. Pat. Nos. 3,969,567; 4,101,335 and 4,428,275 are incorporated herein by reference.
The Hatschek method used in the present invention is disclosed in the book ASBESTZEMENT by H. Klos, published by Springer-Verlag (1967), pp. 113-199, particularly p. 113.
The state of the art of air hardening by being stored in stacking structures may be ascertained by reference to Ullmann's ENCYCLOPEDIA OF TECHNICAL CHEMISTRY, Volume 4, (1953), pp. 232 and 233, and ASBESTZEMENT TECHNOLOGIE UND PROJEKTIERUNG, published by Springer-Verlag (1967), pp. 113 , particularly 119.
When cement is used as the binder, there is a danger that over an extended period of time the calcium-sulfate/aluminum-hydrate (ettringite) is formed, which in the long run destroys the cellulose fibers.
For that reason attempts already have been made to replace the cellulose fibers with plastic fibers, illustratively polyvinyl alcohol (PVA) fibers or polyacrylonitrile (PAN) fibers. However, plastic fibers of this type are expensive compared to cellulose fibers and thereby the reinforced panels become uneconomical.